1. Field of Invention
The present invention is generally directed to devices for optical communications, and more particularly to a method for making photosensitive fiber suitable for wavelength stabilization gratings.
2. Description of the Related Art
Wavelength stabilization gratings (also referred to as laser stabilization gratings) are weak fiber Bragg gratings used to lock a semiconductor laser to a particular emission wavelength. They are typically a few tenths of a nanometer wide and reflect a small percentage of the incident light, e.g. 1 to 10% of the guided power in the fiber.
Generally, wavelength stabilization gratings are fabricated using UV-induced index changes in a host fiber, such as a standard telecommunications type fiber. For 980-nm pump laser applications, these gratings are typically written in a 980-type fiber, such as Corning(copyright) CS-980(trademark) fiber or Corning(copyright) Flexcor(trademark) 1060 fiber. For 1480-nm pump laser applications, Raman amplifier pump applications, or signal laser applications, they may be fabricated in a fiber such as Corning(copyright) SMF-28(trademark) fiber. Additionally, gratings may be written in a polarization-maintaining (PM) fiber such as Corning(copyright) PureMode(trademark) PM Engineered fiber, PM 980 or PM 1550.
Hydrogen loading must be used with in standard telecommunications type fibers to get the change in index required to make wavelength stabilization gratings. Hydrogen loading involves placing fibers in a chamber pressurized with hydrogen for extended periods of time at very high pressures, e.g., up to 12 days at 20-750 atm pressure. The loaded fibers are then stored at very cold (e.g. xe2x88x9280xc2x0 C.) temperatures to prevent outdiffusion of H2. After grating formation, the gratings are thermally annealed to stabilize the index change, typically with a 24 hour anneal cycle at 140xc2x0 C.
It would be preferable to remove the need for hydrogen loading. Hydrogen loading introduces several processing steps which may be preferable to avoid in manufacturing situations. This leads to increased manufacturing costs due to: increased processing steps, pressure chamber safety concerns, processing variability due to hydrogen out-diffusion, low temperature storage cost, and annealing requirements, among others. In addition, hydrogen loading is a long and expensive process. Therefore, it would be desirable to have a method of fabricating wavelength stabilization gratings which does not require hydrogen loading.
The present invention includes a method of making a preform for an enhanced photosensitive fiber comprising the steps of depositing successive layers of optical material inside a tube using modified chemical vapor deposition, and collapsing the successive layers of optical material in a reducing atmosphere with a positive pressure. Preferably, the positive pressure is between 0 and 1.0 torr. Additionally, the reducing atmosphere preferably comprises He.
The present invention also includes a method of making an enhanced photosensitive fiber comprising the steps of making a preform using modified chemical vapor deposition wherein the preform is collapsed in a reducing atmosphere with a positive pressure and drawing the preform into a fiber. Preferably, the positive pressure is between 0 torr and 1.0 torr. Additionally, the draw tension is preferably between 100 g and 250 g and the draw temperature is preferably between 1950xc2x0 C. and 2100xc2x0 C.